A number of liquefaction systems for cooling, liquefying, and optionally sub-cooling natural gas are well known in the art, such as the single mixed refrigerant (SMR) cycle, propane pre-cooled mixed refrigerant (C3MR) cycle, dual mixed refrigerant (DMR) cycle, C3MR-Nitrogen hybrid (such as the AP-X® process) cycles, nitrogen or methane expander cycle, and cascade cycles. Typically, in such systems, natural gas is cooled, liquefied, and optionally sub-cooled by indirect heat exchange with one or more refrigerants. A variety of refrigerants might be employed, such as mixed refrigerants, pure components, two-phase refrigerants, gas phase refrigerants, etc. Mixed refrigerants (MR), which are a mixture of nitrogen, methane, ethane/ethylene, propane, butanes, and optionally pentanes, have been used in many base-load liquefied natural gas (LNG) plants. The composition of the MR stream is typically optimized based on the feed gas composition and operating conditions.
The refrigerant is circulated in a refrigerant circuit that includes one or more heat exchangers and one or more refrigerant compression systems. The refrigerant circuit may be closed-loop or open-loop. Natural gas is cooled, liquefied, and/or sub-cooled by indirect heat exchange against the refrigerants in the heat exchangers.
Each refrigerant compression system includes a compression circuit for compressing and cooling the circulating refrigerant, and a driver assembly to provide the power needed to drive the compressors. The refrigerant is compressed to high pressure and cooled prior to expansion in order to produce a cold low pressure refrigerant stream that provides the heat duty necessary to cool, liquefy, and optionally sub-cool the natural gas.
Various heat exchangers may be employed for natural gas cooling and liquefaction service. Coil Wound Heat Exchangers (CWHEs) are often employed for natural gas liquefaction. CWHEs typically contain helically wound tube bundles housed within an aluminum or stainless steel pressurized shell. For LNG service, a typical CWHE includes multiple tube bundles, each having several tube circuits.
In a natural gas liquefaction process, natural gas is typically pre-treated to remove impurities such as water, mercury, acid gases, sulfur-containing compounds, heavy hydrocarbons, etc. The purified natural gas is optionally precooled prior to liquefaction to produce LNG.
Prior to normal operation of the plant, all the unit operations in the plant need to be commissioned. This includes start-up of natural gas pretreatment process if present, refrigerant compressors, pre-cooling and liquefaction heat exchangers, and other units. The first time a plant is started up is hereafter referred to as “initial start-up.” The temperature that each portion of a heat exchanger operates at during normal operation is referred to as the “normal operating temperature.” The normal operating temperature of a heat exchanger typically has a profile with the warm end having the highest temperature and the cold end having the lowest temperature. The normal operating temperature of a pre-cooling heat exchanger at its cold end and a liquefaction exchanger at its warm end is typically between −10 degrees C. and −60 degrees C. depending on the type of pre-cooling refrigerant employed. In the absence of pre-cooling, the normal operating temperature of a liquefaction heat exchanger at its warm end is near ambient temperature. The normal operating temperature of a liquefaction heat exchangers at its cold end is typically between −100 degrees C. and −165 degrees C., depending on the refrigerant employed. Therefore, initial start-up of these types of exchangers involves cooling the cold end from ambient temperature (or pre-cooling temperature) to normal operating temperature and establishing proper spatial temperature profiles for subsequent production ramp-up and normal operations.
An important consideration while starting up pre-cooling and liquefaction heat exchangers is that they must be cooled down in a gradual and controlled manner to prevent thermal stresses to the heat exchangers. It is desirable that the rate of change in temperature, as well as the temperature difference between hot and cold streams within the exchanger are within acceptable limits. This temperature difference could be measured between a specific hot stream and a cold stream. Not doing so may cause thermal stresses to the heat exchangers that can impact mechanical integrity, and overall life of the heat exchangers that may eventually lead to undesirable plant shutdown, lower plant availability, and increased cost. Therefore, care must be taken to ensure that heat exchanger cool-down is performed in a gradual and controlled manner.
The need to start-up the heat exchangers may also be present after the initial start-up of the plant, for instance during restart of the heat exchangers following a temporary plant shutdown or trip. In such a scenario, the heat exchanger may be warmed up from ambient temperature, hereafter referred to as “warm restart” or from an intermediate temperature between the normal operating temperature and ambient temperature, hereafter referred to as “cold restart.” Both cold and warm restarts must also be performed in a gradual and controlled manner. The terms “cool-down” and “start-up” generally refer to heat exchanger cool-down during initial start-ups, cold restarts as well as warm restarts. FIG. 9 shows exemplary temperature profiles of a heat exchanger before and after a warm restart. FIG. 10 shows exemplary temperature profiles of a heat exchanger before and after a cold restart.
One approach is to manually control the heat exchanger cool-down process. The refrigerant flow rates and composition are manually adjusted in a step-by-step manner to cool down the heat exchangers. This process requires heightened operator attention and skill, which may be challenging to achieve in new facilities and facilities with high operator turnover rate. Any error on the part of the operator could lead to cool down-rate exceeding allowable limits and undesirable thermal stresses to the heat exchangers. Additionally, in the process, the rate of change of temperature is often manually calculated and may not be accurate. Further, manual start-up tends to be a step-by-step process and often involves corrective operations, and therefore is time consuming. During this period of start-up, feed natural gas from the exchanger is typically flared since it does not meet product requirements or cannot be admitted to the LNG tank. Therefore, a manual cool-down process would lead to large loss of valuable feed natural gas.
Another approach is to automate the cool-down process with a programmable controller. However, the approaches disclosed in the prior art are overly complicated and do not involve feed valve manipulations until the exchanger has already cooled down. This can easily lead to a large oversupply of refrigerant in the heat exchanger and would be inefficient. In the case of a two-phase refrigerant such as mixed refrigerant (MR), this could lead to liquid refrigerant at the suction of the MR compressor. Additionally, this method does not take advantage of the close interactions between the feed flow rate and refrigerant flow rate, which have a direct impact on hot and cold side temperatures. Finally, this method is rather an interactive (not automatic) process with the crucial decisions still having to be made by the operator. Its level of automation is limited.
Once the LNG plant has started up, various control schemes such as those described in U.S. Pat. Nos. 5,791,160 or 4,809,154 may be utilized to control parameters such as the LNG temperature, flow rate, heat exchanger temperature differences and so on. Such control schemes are different from those utilized during start-up and cannot be readily used for start-up purposes. Firstly, the temperature profiles are already established and are to be maintained relatively stable and feed gas and refrigerant flow rate do not need to be increased from zero as in the case of start-up. This eliminates one critical variable in the control scheme. Additionally, during normal operation, refrigerant composition may require no or small adjustments, unlike during start-up where larger adjustments need to be made throughout the start-up process. In the case of mixed refrigerant processes, refrigerant component inventory may not be available during start-up which further complicates the control process. Further, refrigerant compressors are often operating in recycle mode during start-up to prevent reaching the surge limit. These recycle valves may need to be gradually closed during the cool-down process, which is an additional variable to be adjusted. Furthermore, during start-up and heat exchanger cool down, the suction pressure needs to be monitored and refrigerant components (such as methane in the case of MR based process and N2 in N2 recycle process) need to be replenished in order to maintain a proper suction pressure. This also complicates the start-up operation.
One potential way to automate the cool down process would be to increase the natural gas feed flow rate while independently manipulating the refrigerant flow rate to control the cooldown rate as measured at the cold end of the heat exchanger. This method is found to be ineffective, because the cool down rate controller can have different and even reverse responses depending on the temperature and phase behavior of the refrigerant. The refrigerant not only serves as a cooling medium, but also a heat load in the heat exchanger before JT valve expansion. At the beginning of the process, increasing the refrigerant flowrate may cause the cooldown rate as measured at the cold end to actually slow before the refrigerant condenses in the tube circuit. Later in the cooldown process when the refrigerant entering the JT valve is condensed, increasing the flow increases the cool down rate. This reverse response makes the automation of such a control method very difficult or infeasible.
Overall, what is needed is a simple, efficient, and automated system and method for the start-up of heat exchangers in a natural gas liquefaction facility, while minimizing operator intervention.